The present invention relates to a motor control device for three-phase brushless DC motors used in assist motors in electric power steering devices. More specifically, the present invention relates to a motor control device that sets a dead time value actively so that dead time is minimized in order to improve the motor""s power supply usage efficiency, the controllability, and to reduce noise.
Three-phase brushless motors (hereinafter referred to as motors) with U-, V-, and W-phases that are easy to control and provide high torque relatively quickly are used, e.g., as assist motors in electric power steering devices. A control device for this type of motor is equipped with a transistor inverter having switching means formed from power MOSFETs (Metal Oxide Semiconductor Field Effect Transistor) in a motor drive device. This motor drive device is formed with high-level and low-stage FETs for each of the U-, V-, and W- phases. The FETs are switched on in an alternating manner by the motor control device, which is equipped with a CPU, thus improving power supply usage efficiency and also providing smooth drive. However, if both a high-level FET and a low-stage FET are on at the same time for some reason, a through current flows through the high-level FET and the low-stage FET for that phase without flowing through the motor, resulting in a circuit that goes directly from the power supply to a ground GRD. Since the current does not pass through the coils of the motor, which have a high resistance, the resistance is low and the current is high, leading to damage to the FETs. Therefore, when a FET is to be turned on, a delay circuit is used to provide dead time so that a predetermined time elapses after the other FET is turned off before the FET is turned on, thus protecting the FETs.
However, two similar power MOSFETs can have varying characteristics. The differing characteristics can be response time and signal rise time and the like. To take all possibilities into account by providing leeway with a high dead time results in reduced power supply usage efficiency as well as contributing to non-linearity in the motor drive signal, which can lead to torque ripple that makes operation difficult and noise.
Dead time correction is one approach to correcting non-linearity caused by dead time. Although this involves complex operations, the processing is static and therefore cannot respond appropriately to changes over time and temperature changes.
In order to overcome these problems, the object of the present invention is to provide a motor control device that allows quick discovery of through-current during dead time even if, besides static tolerances such as production variations between switching means, there are differences caused by dynamic changes such as temperature changes and changes over time. Another object of the present invention is to provide a motor control device in which dead time is set in an active manner to minimize dead time and non-linearity caused by variations in switching means characteristics are minimized to improve motor efficiency and responsiveness while reducing noise.
A motor control device according to claim 1 includes: high-stage FETs disposed on a power-supply side and low-stage FETs disposed on a ground side, the FETs being switching means connected in series in circuits disposed between an application point of a power supply and a ground point and associated with U-, V-, and W-phases of a brushless DC motor, the FETs being selected in an exclusive manner; connection points disposed between the high-level FETs and the low-stage FETs supplying drive currents to U-, V-, and W-phase motor coils based on combinations of open/close states of the high-level FETs and the low-stage FETs for each phase; current detecting means detecting currents in the circuits; and switch controlling means controlling switching means and driving the brushless DC motor. Means for detecting irregular current detecting current at a predetermined timing using current detecting means. Means for evaluating motor drive circuit irregularities evaluates motor drive circuit irregularities based on current values detected by irregular current detecting means.
In a motor control device according to this structure, irregularities in the motor drive circuit are detected immediately by motor drive circuit irregularity evaluating means based on the current value detected by irregular current detecting means.
In addition to the structure of the motor control device described in claim 1, the motor control device according to claim 2 also includes correcting means correcting a dead time setting based on a current value detected by irregular current detecting means.
In addition to the operations provided by the motor control device described in claim 1, this motor control device uses correcting means to correct dead time settings based on a current value detected by irregular current detecting means when motor drive circuit irregularity evaluating means detects and irregularity.
In addition to the structure of the motor control device described in claim 1, in the motor control device according to claim 3, irregular current detecting means detects a trailing edge when a gate signal for the U-, V-, or W-phase goes from on to off or a leading edge going from off to on and detects through-current during a dead time for each phase with current detecting means using this timing as a reference.
In addition to the operations provided by the motor control device described in claim 1, this motor control device detects a trailing edge when a gate signal for the U-, V-, or W-phase goes from on to off or a leading edge going from off to on. Through-current is checked at a timing appropriate for each phase, thus allowing precise control.
In addition to the structure of the motor control device described in claim 1, in the motor control device according to claim 4, current detecting means is disposed so that current flowing through the low-stage FETs is detected. Irregular current detecting means detects when controlling means is sending xe2x80x9conxe2x80x9d gate signals to U-, V-, and W-phase high-stage FETs while sending xe2x80x9coffxe2x80x9d gate signals to all low-stage FETs and detects through-current in the phases at this timing using current detecting means.
In addition to the operations provided by the motor control device described in claim 1, this motor control device checks for through-current when U-, V-, W-phase low-stage FET gate signals are all off. Thus, irregularities can be detected by performing current detection on just one position, allowing a simple structure.
In addition to the structure of the motor control device described in claim 1, in the motor control device according to claim 5, current detecting means is disposed so that current flowing through the low-stage FETs is detected. Irregular current detecting means detects when controlling means is sending xe2x80x9conxe2x80x9d gate signals to U-, V-, and W-phase low-stage FETs while sending xe2x80x9coffxe2x80x9d gate signals to all high-stage FETs and detects through-current in the phases at this timing using current detecting means.
In addition to the operations provided by the motor control device described in claim 1, this motor control device checks for through-current when U-, V-, W-phase high-stage FET gate signals are all off. Thus, irregularities can be detected by performing current detection on just one position, allowing a simple structure.
In addition to the structure of the motor control device described in claim 2 or claim 3, in the motor control device according to claim 6 correcting means reduces by a predetermined amount a dead time setting set up based on a turn-off dead time and a turn-on dead time set up ahead of time if a predetermined through-current is not detected by irregular current detecting means. If the predetermined through-current is detected, the correcting means increases the dead time by a predetermined amount.
In addition to the operations provided by the motor control device described in claim 2 or claim 3, this motor control device can reduce dead time if no through-current is flowing. This allows power supply usage to be improved and torque ripple and noise to be reduced. If through-current is flowing, the turn-off dead time and the turn-on dead time are reset to extend the dead time so that the FETs can be safely protected.
In addition to the structure of the motor control device described in claim 6, in the motor control device according to claim 7, if the through current is detected at a predetermined dead time value, correcting means does not reduce the dead time below the dead time value at which the through-current was detected.
In addition to the operations provided by the motor control device described in claim 6, when a through-current is detected at a predetermined dead time setting, correcting means does not reduce the setting below the dead time setting at which the through-current was detected. This safely protects the FETs.
In addition to the structure of the motor control device described in claim 6 or claim 7, in the motor control device according to claim 8 controlling means sets dead time values independently for each of the phases.
In addition to the operations provided by the motor control device described in claim 6 or claim 7, dead time is set independently for each phase. Thus, optimal settings that correspond to variations in individual FETs can be provided.
In addition to the structure of the motor control device described in any one of claim 6 through claim 8, in the motor control device according to claim 9, correcting means sets the dead time setting value independently of turn-off dead time and turn-on dead time.
In addition to the operations provided by the motor control device described in any one of claim 6 through claim 8, the dead time is set independently for the turn-off dead time and the turn-on dead time. Thus, optimal settings corresponding to variations in individual FETs can be provided.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.